I'm thinking of some effect that correlates with the direction of the telescope. For example, if it turned out that quasars with strong jets had magnetospheres with charged particles caused the lines to drift, and it so happens because you are looking in different directions you are more likely to see quasars with strong jets because those with weak ones are more likely to get obscured by interstellar dust.Still, what such uncertainty would be explain why the data set from either telescope separately gives the same direction for the dipole?
Or it turns out that when they did the star catalogs that they did them in a way that certain types of quasars are preferred in one part of the sky and not in others.
Or the local ISM. You said yourself that dipoles are usually a sign of something changing at much greater scales that your observational volume. If your observational volume is the observable universe, you have something hard to explain. If it turns out that what you are seeing is nearby, it's much less hard to explain.Do you think it is an artifact of the milky way?
I think they've done a reasonable job of making sure that their result isn't equipment related, and that's important, because if it turns out that there is some unknown local ISM effect that changes quasar line widths in odd ways, that's still pretty interesting physics.